1. Field of the Invention
The invention relates to a resonant circuit and, in particular, to a resonant circuit used in a power converter.
2. Description of Related Art
With reference to FIG. 24, the U.S. Pat. No. 6,344,979, entitled “LLC series resonant DC-to-DC converter” has disclosed a LLC resonant power converter. The power converter mainly includes a square wave generator 100, a transformer 110, a resonant unit 120, a rectifying unit 130 and a filtering unit 140. The resonant unit 120 is connected between the primary side of the transformer 110 and the output terminal of the square wave generator 100. The rectifying unit is connected to the secondary side of the transformer 110. The filtering unit 140 is connected to the output terminal of the rectifying unit 130 for connecting to a load 150.
The resonant unit 120 has a series capacitor Cr connected with a series inductor Ls in series. Both the series capacitor Cr and the series inductor Ls are further connected with an inductor Lm provided by the transformer 110 in parallel. A first characteristic resonant frequency fr of the resonant unit 120 is determined by the series capacitor Cr and the series inductor Ls together. A second characteristic resonant frequency fm is determined by the series capacitor Cr, the series inductor Ls, and the inductor Lm altogether.
With reference to FIG. 25, the U.S. published application No. 2005/0207180 entitled “LLC half-bridge converter” discloses another resonant power converter. The power converter has a basic configuration and circuit operations being similar to the above-mentioned circuit. However, it can enhance the conversion efficiency of the converter. The transformer 110 provides two primary windings and two secondary windings. Two inductors Lm1 and Lm2 provided by the two primary windings are connected in series, whereas the two secondary windings are connected in parallel.
Either of the above-mentioned circuits uses the LLC resonant design. An obvious drawback of the LLC circuit is that the resonant power converter has to be operated in a wider operating frequency bandwidth in order to adjust its output voltage. Only by this can the circuit provide a stable operating voltage in a range to a load. In cases where a wider operating frequency bandwidth is impossible, it will be difficult for the LLC resonant design to effectively make the conversion or it simply reduces its conversion efficiency.